Crisis in Cosmology? Or Turning Point?

Like the 19th century American frontier, contemporary cosmology can sometimes
be wild and woolly. Despite--and sometimes even because of-- recent
advances in
astronomy, the data don't always agree with prevailing theories about the
universe. Numerical cosmologists, reliant upon both observation and theory,
join the fray as they struggle to answer a number of fundamental questions
about the universe: What is the universe made of? How old is it? What is its
destiny?

Missing Mass: the Mystery Continues

Inflation and the age of the universe predicts that the shape
and fate of the universe is bound up with the average density of the
matter it contains. If this average density exactly equals the critical density, the density required
to arrest the infinite expansion of the universe but without bringing about its ultimate collapse, then
Omega equals 1, and we live in a flat universe.

Astronomers have managed to see, at most, one-tenth of the matter
necessary to close the universe.
This, along with the relative motions of
galaxies and galaxy clusters, even
superclusters, points to the
existence of dark matter--a lot of it,
perhaps 90% of what's out there.

Perhaps the weakest part of our current theories of cosmological structure
formation is that we don't know what dark matter is made of. Some would
say, we don't even know that it exists, although there are powerful
observational and theoretical clues that indicates that its there. Dark
matter may take several forms, both baryonic and nonbaryonic--an idea that
many purists find repugnant.

Baryonic dark matter simply fails to emit radiation
detectable on earth, yet it is generally believed to exist to make up the deficit between the amount
predicted from light element nucleosynthesis measurements and that visible
in galaxies. Baryonic dark matter in the form of dim stars or more
generally MACHOS will continue to be inventoried with every more sensitive
telescopes, although it is likely much of it will remain too dim to be
detected.

The apparent scarcity of such baryonic dark matter
has led scientists to explore other, more exotic types of matter: exotic,
non-baryonic particles. They come in two flavors: cold dark matter, or WIMPS
(Weakly Interacting Massive Particles); or hot dark matter, the most likely
candidate being the massive neutrino. But WIMPS are largely hypothetical and the
evidence for neutrinos with mass is less than rock solid.

Neutrinos--the one kind of nonbaryonic dark
matter we know exists--has a devilishly difficult mass to measure.
Experiments with particle accelerators will continue to try to pin down the
mass. If it is conclusively found to be non-zero, then neutrinos will make
an important contribution to cosmic dark matter.

The nature of cold dark matter--the ingredient that
seems to be required to make a successful cosmological model--therefore remains a
mystery. If it is composed of WIMPS, then their detection in the laboratory
would be a major step forward for the field, putting the entire class of
dark matter-dominated models on a firm physical footing.

All of these uncertainties force numerical cosmologists to make some critical
assumptions about the universe. How much baryonic, cold and dark matter
should
be included in the simulations, and what should they be? Is Omega in fact
equal
to 1? Some cosmologists think that the evidence points toward a less
dense--and
therefore open--universe.

Grand
Challenge Cosmology Consortium members
Renyue Cen and Jeremiah Ostriker believe that the only way to simulate the
evolution of the universe is to decrease the percentage of dark matter. And,
since the amount of ordinary baryonic matter is a known quantity, that means
decreasing the total density of matter--perhaps to only one-third of that required
for a closed universe. That, of course, would mean that the Standard Big Bang model (plus inflation) could be in serious trouble.

The search for dark matter will likely intensify in the coming decade. The most exciting
prospect is that in the process of looking for dark matter, something
completely unexpected will be found which will alter our view of our
universe.

The Question of Age

The age of the universe is another, equally vexing question. Astronomers have
known for some time that only two pieces of information are needed to
calculate
the age of the universe: the velocity at which far-off galaxies are receding from us and their distance.
From the ratio of these two numbers, the Hubble constant, cosmologists can
determine how fast the universe is expanding and, by extension, the age
of the
universe.

Until recently, most estimates of the Hubble constant were
around 50,
putting the age of the universe at about about 20 billion years. But recent
distance measurements using the Hubble Space Telescope and the Keck telescope
atop Hawaii's Mauna Kea indicate that the universe might be considerably
younger--about eight to twelve billion years old.

These new estimates present considerable problems to cosmologists.
Astronomers
who study the chemistry and life cycles of stars calculate, with considerable
confidence, that the oldest stars in the Milky Way are about 14 billion years
old. Clearly, something is missing from the picture.

The age problem is only confounded by the search for missing dark matter: if
Omega is equal to 1, as inflation theory requires, then the gravitational
force of all that mass would tend to slow down expansion, making the universe even
younger than it now appears.

Einstein may yet save the day with his ill-fated cosmological constant.
Because
the prevailing view in 1916 was that the universe was static, he was dismayed
to find that his theory of General Relativity predicted either an
expanding or
contracting universe, and introduced a fudge factor to his equations to make
his calculations consistent with a static universe. This "cosmological
constant" represented a repulsive force of energy equal to but opposite
that of
gravity. Einstein later dropped the cosmological constant when Hubble showed
that the universe was indeed expanding, calling it "the greatest blunder
of my life."

But now Einstein's blunder is gaining some cachet within cosmological
circles,
because the force, if it did (or does) exist, solves the age-missing mass
crisis neatly. Inflation would still occur, but there would follow a long
period of very leisurely expansion, giving stars and galaxies lots of
time to
form. At some point, the cosmological constant would take effect, increasing
the rate of expansion with its anti-gravitational force. The current rate of
expansion would no longer be a reliable indicator for the pre-cosmological
constant rate of expansion. However, explaining the physical basis for the cosmological constant poses yet another challenge to the theorists.

Numerical cosmologists are confronted by yet another set of variables: How should
they place their estimates on the age of the universe? Should they
introduce a
cosmological constant? If so, what should the value be? When would it kick
in?

Top-down versus Bottom-up

The Grand Challenge
Cosmology Consortium's simulations
have assumed that large-scale structure evolved from the
"bottom-up"--that is,
small-scale structures such as galaxies formed first, only later merging to
form the vast sheets and filaments that we observe today. But "bottom-up"
simulations have a problem: they yield primarily filamentous large-scale
structures, and not the sheet-like structures
Geller
and others have observed.

There is another approach to the question of large scale structure formation,
however: the "top-down" theory, proposed by the Russian physicist Yakov
Zel'dovich in the 1970s. This theory proposes that large-scale
density
fluctuations caused vast, pancake-like structures to form first. The
pancakes
eventually fragmented into galaxies and galaxy clusters.

Although the "top-down" theory has of late fallen out of favor, it may be enjoying something of a
renaissance with a model, recently described by a cosmologist from the Institute for
Astronomy at the University of Hawaii, which marries top-down with bottom-up
theory. The marriage allows for hierarchical clustering at smaller scales
within a Zel'dovich scenario operating at much larger scales.